There are other reasons that a probe might lack a color camera. For example, if the color camera is even just a few ounces heavier than a black-and-white camera, the cost of extra fuel to get it into space might make it too expensive for the mission.

Although I seem to remember that in the case of the Mercury Messenger, they compromised by equipping it with a black-and-white camera and a few interchangeable color-filters. So that probe can send color pictures home, but it requires three passes over the target to do so.